A membrane switch is a momentary switch device having at least one contact that is provided on or made of a flexible substrate. Membrane switches are commonly used in computer keyboards and myriad other applications. In a conventional membrane switch arrangement, three layers of thin polyester (e.g., Mylar.TM.) sheets are sandwiched together. Membrane switches may be constructed with a rigid layer, e.g., a layer of glass, plastic, or printed circuit board material, as used in a touch panel membrane switch, touchpad or keypad. The outer two layers each carry on their opposing inside surfaces conductive switch circuit patterns. An intermediate layer is disposed between the outer two layers and acts to isolate the switch circuit patterns from each other. The switch circuit patterns may, however, be selectively brought into electrical contact with each other through contact apertures provided in the intermediate layer. When selective contact is made between the opposing switch circuit patterns, an electrical circuit may be completed to perform a predetermined action.
Referring to FIG. 1, a conventional computer keyboard 100 may utilize a conventional membrane switch structure 200, as shown in FIG. 2, which is installed underneath a set of keyboard keys 101, to convert key selections into electrical signals representing corresponding alphanumeric characters or functions. Membrane switch structure 200 generally has three layers 201, 202 and 203. Outer layers 201 and 202 sandwich intermediate layer 203. The three layers are generally made out of a thin insulative sheet, for example, polyester, glass, plastic or even common printed circuit board material. Outer layers 201 and 202 each have, on respective opposing inside surfaces 210 and 211, switch circuit patterns (208 and 209, respectively), which may be deposited by print transfer (e.g., silk screening), spraying or other techniques. The circuit patterns may be printed with suitable conductive inks, e.g., a polymer-based conductive ink having silver, carbon, and/or other conductive particles in suspension. Typically, each keyboard key is coupled to a flexible plunger positioned to make contact with a backside of an upper one of the two outer layers of the membrane switch. Depression of a selected keyboard key 101 causes a corresponding plunger to exert pressure on the upper outer layer of the membrane switch. The resulting pressure causes an electrical circuit printed on the inner face of the top outer layer to come resiliently into electrical contact with a corresponding circuit printed on the inner face of the bottom outer layer of the membrane switch, through contact apertures provided in the intermediate layer. The electrical contact results in generation of a signal input to an integrated circuit (IC) located within the keyboard. The IC, in turn, provides a digital output signal readable by the associated computer.
Circuit patterns 208 and 209 are appropriately laid out to provide contact points and lines of conduction for each of keyboard keys 101 within a conventional switch matrix. Thus, e.g., by key depression at location 212, upper membrane switch circuit 208 may be brought into electrical contact with the lower membrane switch circuit 209, through contact aperture 204 in intermediate layer 203. The electrical contact between the designated contact points of opposing circuits 208 and 209 results in generation of an electrical signal (high or low voltage) on a particular line of the keyboard switch matrix. By recognizing the line on which the signal is generated, and the timing thereof, the keyboard mounted IC can discriminate which of keyboard keys 101 has been depressed.
Membrane switch circuitry tends to be susceptible to disturbance by electrostatic discharge (ESD). ESD events can induce noise voltages and currents on the circuits of the membrane switch resulting in device operation problems. Various schemes are known which serve to reduce the ESD susceptibility of membrane switches. Most conventionally, a metal grounding plate may be positioned under an entire switch circuit pattern for providing ESD protection. Although a metal grounding plate can shield the associated switch circuitry from ESD events, use of a metal grounding plate can significantly increase keyboard fabrication costs and product weight. A second known approach involves use of an insulative sheet with a layer of continuous conductive material printed thereon. The printed sheet emulates the presence of the metal grounding plate at a lower cost. While a continuous layer of conductive material on an insulating sheet is less expensive than a metal grounding plate, there is significant cost associated with the conductive ink required to produce the continuous conductive layer. Another known approach is to print on an insulating sheet a rectilinear conductive grid pattern 301 (see FIG. 3), or a concentric circle grid pattern 401 (see FIG. 4). Using these types of grid patterns can reduce the costs associated with a continuous layer of conductive ink. However, depending on the spacing and location of the gridlines relative to the lines of the switch circuit patterns, the grid patterns do not necessarily ensure that all lines of the switch circuit patterns are adequately protected from ESD. Increased ESD protection can be achieved by increasing the density of the grid lines, but only at a higher cost approaching the cost of a continuous conductive layer.
It would be desirable to have a membrane switch configuration providing optimized ESD protection at reduced costs.